To understand the causes and consequences of differences across physicians, physician groups, and hospitals in the efficiency with which they deliver medical care, it is crucial to have a comprehensive database with sufficient power to characterize detailed patterns of health care utilization and to be able to link those files to supplemental data now available from other sources such as patient or provider surveys. The Medicare claims data are ideal for this task because of three key features. First, they are national in scope: 98% of the elderly are eligible for Medicare, and because almost all hospitals and physicians provide care to Medicare enrollees, the data can provide insight into factors that influence both hospital performance and physician practice more generally. Second, the claims data provides detailed demographic and financial information, as well as rich clinical detail on the specific services provided to fee-for-service enrollees. Third, with appropriate approvals to preserve confidentiality, the data can be obtained with individual identifiers that can support linkage across files, over time, and to other sources. The utility of these data for a wide range of health care and economic research studies has been amply demonstrated. The complexity and costs of analyzing these data motivate the specific aims of this Data Core: 1. To maintain and update a comprehensive and secure database of Medicare enrollment files, claims records, and supplementary files. 2. To obtain and manage specific research files required for this Program Project Grant and to develop project-specific analytic files. 3. To maintain a computing infrastructure and procedures capable of managing a high volume of patientidentifiable, confidential data while ensuring timely and appropriate access to authorized investigators. 4. To make the research files developed under this program project grant available to the research community to the extent permissible under law. Comprehensive information on the specific services provided to Medicare beneficiaries and the quality and costs of the care they receive can provide a powerful tool for improving the efficiency of medical care. The confidential and personal nature of these databases mandates that they be maintained in a secure and efficient data center that can manage them effectively while protecting the integrity of the data and supporting the needs of authorized investigators.